Band pass amplifier



Jan. 16, 1934. R BRADEN BAND PAS S AMPLIFI ER Filed Dec. 3, 1929 MPO) INVENTOR RENE A. BRADEN BY 2% ATTORNEY UNET STATES PATENT j' 1,943,416 BAND PASS AMPLIFIER Rene A. Braden, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 3, 1929. Serial- No. 411,261

9 Claims. (01. 178-44.)

My present invention relates to radio frequency Other objects of the invention are to improve amplifiers, and, more particularly, to an ampligenerally the simplicity and efficiency of coupling fier adapted to pass a band of radio frequencies controlling means for band pass amplifiers, and of a substantially constant width throughout a to provide a tuned radio frequency receiver empredetermined range of frequencies. ploying a band pass amplifier which is reliable in 60 In co-pending applications I have disclosed operation and economically manufactured. different arrangements of band pass amplifiers The novel features which I believe to be charfor use with tuned radio frequency receivers. In acteristic of my invention are set forth in particueach of these applications the basic conception larity in the appended claims, the invention itself,

0 has been the amplification of radio frequency however, as to both its organization and method 65 currents by coupling the primary and secondary of operation will best be understood by reference circuits of a band pass amplifier in such a way to the following description'taken in connection that the accepted band width remained substanwith the drawing in which I have indicated diatially constant when the amplifier was tuned grammatically several circuit organizations to various wave frequencies. It was also shown whereby my invention may be carried into effect. 70

in the aforementioned applications that the es- In the drawing, sential requirement for constant accepted band Fig. 1 shows diagrammatically an arrangement width was that the coupling between the primary embodying the invention, and secondary circuits vary so as to be inversely Fig. 2 shows a modified arrangement,

proportional to the wave frequency. Various Fig. 3 shows an arrangement which is a modi- 75 novel methods and arrangements for controlling fication of Fig. 2. the coupling between primary and secondary Referring to the accompanying drawing in tuned circuits were presented in these applicawhich like characters of reference indicate the tions, all of these arrangements having as their same parts in, the different figures, there is disprime consideration the accomplishment of the closed in Fig. 1 one arrangement embodying the 0 result hitherto referred to. present invention. An electron discharge device After further experimentation I have diseov- 1, of either the three electrode or four electrode cred additional methods of, and devised further type, has radio frequency currents impressed arrangements for, securing a substantially conupon its intput circuit, the output circuit of the stant accepted band width in a band pass amplidevice being tuned and coupled to the tuned input 5 her. Basically considered, the new methods incircuit of a second electron discharge device 8, volve the controlling of the co-efficient of couwhich latter device may be of the three or four pling between tuned radio frequency circuits by electrode type. diverting coupling current with increasing The output circuit of the device 1 includes an frequencies, inductance coil 3 which is tuned by the variable 90 It may be stated that one of the main objects condenser 22, while the input circuit of the device of the present invention is to provide a method 8 includes an inductance coil 6 tuned by the variof operating a band pass amplifier, which ampliable condenser "1. Each of the tuned circuits also fier includes at least a pair of tuned oscillation includes a coupling coil, the coil 4 being connected circuits, consisting in magnetically coupling the in series with the coil 3, and inductively coupled, 95 tuned circuits, and shunting current from the as at M, with the coupling coil 5, in series with coupling between the circuits at high frequencies the inductance coil 6. A resistance 9 is connected in such a manner that the effective coupling bein shunt with the coupling coil 4, while a resisttween the tuned circuits varies inversely with ance 10 is connected in shunt with the coil 5.

the change in wave frequencies. Without the resistors 9 and 10, the whole cur- 100 Another important object is to provide a tuned rent of the tuned circuits would flow through the radio frequency receiver employing a band pass two coupling coils 4 and 5, and the coupling would amplifier, which amplifier includes at least a pair remain constant. However, with the resistances 9 of tuned oscillation circuits adapted to amplify and 10 connected as shown, the current through radio frequency currents, each of said circuits the coil 3 is divided. A small portion of the cur- 5 being provided with a coupling unit, whereby the rent flows through the resistance 9, and the tuned circuits are coupled, and means, shunting greater part passes through the coupling coil 4. each of said units, for diverting current from each This division of current occurs, of course, when of the units as the frequency of the currents is the frequency of the currents flowing through increased. the tuned circuits is low.

It will be understood that, in the same manner, the current fiowing through the coil 6 divides between the coupling coil 5 and the resistance 10, when the frequency is low, the greater part fiowing through the coil 5. At high frequencies, however, the reactance of coil 4 becomes much higher than it was at lower frequencies, and a large proportion of the current through the inductance coil 3 flows through the shunting resistance 9, and a smaller proportion through the coupling coil 4. A similar change occurs in the secondary circuit 6, '7, 5 and 10.

It will, therefore, be observed that the effective coupling between the two tuned circuits is reduced at high frequencies in proportion to the shunting of current out of the coupling coils 4 and 5. By proper choice of the shunting resistors 9 and 10, of coupling inductances 4 and 5, and of the mutual inductance M, between the coupling inductances, the coupling between the tuned circuits can be made to vary with frequency in such a manner that the band width will be practically the same at all wave frequencies amplified.

The circuit in Fig. 2 embodies the same elements as described for Fig. 1, with the exception that the coupling coils 4 and 5 are shunted by capacities l1 and 12 respectively, it being understood that the operation of the circuit of Fig. 2 is practically the same as that of Fig. 1. The same principle underlies both circuits, the circuit in Fig. 2, representing a modification which may be desirable under somewhat different circumstances.

I have found that a circuit, as shown in Fig. 3, is more suitable for use with a three electrode vacuum tube than the modification shown in Fig. 2. That is to say, the same coupling varying means is employed in the circuit shown in Fig. 3 for three electrode tubes, as is shown in connected with the four electrode tubes in the circuit of Fig. 2.

As will be noted from Fig. 3, the output circuit of the triode includes an inductance coil 20 which is inductively coupled with an inductance coil 21, the latter being part of a tuned circuit. The latter circuit is tuned by a variable condenser 2 in shunt with the coil 21. The input circuit of the succeeding triode 8 is tuned by a variable condenser '7 in shunt with the inductance coil 22.

The circuit 2, 21, 4, 11 and the tuned input circuit of the triode circuit 8 are inductively coupled by coils 4 and 5. Each of the latter coils has connected in shunt with it a condenser, as shown in Fig. 2. The grid of the tube 8 is connected between the coupling coil 5 and the variable condenser 7. As stated, heretofore, this circuit so far as its constant band width characteristic is concerned, embodies the same means as shown in the modification in Fig. 2, the additional novelty residing in the manner of employing the coupling varying means with a receiver circuit including triodes.

To more clearly explain the difference between Fig. 3 which is more suitable for use with a three electrode vacuum tube, and the circuit in Fig. 2, in Fig. 2 the four electrode tube is connected directly to the tuned circuit 2, 3, 4, 11. This is permissible because the tube has very high internal plate resistance. In Fig. 3, the three electrode tube 1 has comparatively low internal plate resistance, (this is characteristic of three electrode tubes in comparison with four electrode tubes) and it is not permissible to connect this tube directly to the tuned circuit 2, 4, 11, 21, for so. doing would render this tuned circuit inoperative. It

is therefore necessary to couple the tube to the tuned circuits by means of a radio frequency transformer, as is commonly done in radio frequency amplification, employing similar tubes and single tuned circuits in each stage. The transformer may be thought of as matching the impedance of the tube to the impedance of the tuned circuit.

Beyond this arrangement which is used on account of the low impedance of the tube 1, there is no essential difference between the circuits of Fig. 2 andFig. 3. In Fig. 3 the coupling elements 4, 11 and 5, 12 have been placed on the high potential side of the coils 21 and 22, respectively, while in Fig. 2 the corresponding elements are connected to the low potential ends of the coils.

There is no difference in essential operation, it being understood that the arrangement in Fig. 3 is employed, because it makes the diagram clearer and easier to understand.

While I have indicated and described several systems for carrying my invention into effect, it Will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made Without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a pair of oscillation circuits, means for inductively coupling the circuits, variable tuning means in each of said circuits for tuning the circuits to a desired radio frequency, and an impedance in each circuit, inde pendent of the tuning means, of a magnitude such that current is diverted from said coupling means in proportion with increase in frequency of coupling current.

2. In combination, in a band pass radio frequency network, a pair of oscillation circuits adapted to transmit radio frequency currents, variable tuning means in each of said circuits for tuning the circuits to a desired radio frequency, a coupling coil in each of said circuits, said coupling coils being magnetically coupled, and an impedance, shunting each of said coupling coils, of a magnitude such that current is diverted from the coupling coils in proportion as the frequency of the currents is increased.

3. A radio frequency amplifier comprising a pair of multi-electrode tubes, an oscillation circuit, including a variable tuning reactance, connected to the output electrode of one of said tubes, an oscillation circuit including a variable tuning reactance, connected to the input electrodes of the other tube, each of said oscillation circuits including a coupling coil, said coupling coils being inductively coupled, said variable tuning reactances being adjustable to tune said coupled oscillation circuits throughout a predetermined radio frequency range, and an impedance connected in shunt with each of said coupling coils, said impedances being so chosen that the effective coupling between the coupled oscillation circuits is reduced at the high frequency portion of said range in proportion to the shunting of coupling current through said impedances.

4. In combination, in a band pass amplifier, a triode, a tunable output circuit, the anode circuit of the triode and the output circuit being inductively coupled, a second triode having a tunable input circuit, a coupling coil in the output circuit and a similar coupling coil in' the input circuit, the output and input circuits being inductively coupled by said coupling coils, and an impedance, electrically connected with each of said coupling coils, of a magnitude such that increased portions of the coupling currents are diverted from each coupling coil with a rise in the frequency of said currents.

5. In combination, in a band pass radio frequency amplifier, a screen grid tube provided with an output circuit, a second screen grid tube provided with an input circuit, means for coupling the circuits, means for tuning each circuit to a desired radio frequency, and a resistor in each circuit electrically connected to said coupling means, each resistor having a magnitude such that current is variably by-passed from said coupling means as the tuning means is varied.

6. In combination, in a band pass radio frequency amplifier, a screen grid tube provided with an output circuit, a second screen grid tube provided with an input circuit, means for coupling the circuits, means for tuning each circuit to a desired radio frequency, and a capacity in each circuit of a sufiicient magnitude electrically connected to said coupling means in such a manner that current is variably by-passed from said coupling means as the tuning means is varied.

7. In combination, in a band pass radio frequency amplifier, a screen grid tube provided with an output circuit, a second screen grid tube provided with an input circuit, means for coupling the circuits, means for tuning each circuit to a desired radio frequency, and a resistor of a proper value in shunt with the coupling means in each circuit for variably by-passing current from said coupling means as the tuning means is varied.

8. In combination, in a band pass radio frequency amplifier, a screen grid tube provided with an output circuit, a second screen grid tube provided with an input circuit, means for coupling the circuits, means for tuning each circuit to a desired radio frequency, and a condenser of a proper capacity value in shunt with the coupling means in each circuit for variably by-passing current from said coupling means as the tuning means is varied.

9. In combination with a pair of successive screen grid tubes, a tunable band pass network coupling the output electrodes of the first tube to the input electrodes of the second tube, said network comprising a pair of oscillation circuits, each of said circuits including a variable reactance for tuning it through a desired frequency range, reactive means for coupling said circuits, and an impedance in each of said circuits connected across said reactive means, the relative magnitudes of said reactive means and impedances being so chosen that current is diverted from said reactive coupling means in proportion as the frequency of the current increases whereby the efiective coupling between said oscillation circuits varies with frequency to maintain the band width transmitted through said network substantially constant throughout said frequency range.

RENE A. BRADEN. 

